Conventionally, a study for a high electron mobility transistor (HEMT) in which an AlGaN layer and a GaN layer are formed over a substrate by crystal growth, and the GaN layer functions as an electron transit layer has been performed. A band gap of GaN is 3.4 eV, and it is larger than a band gap of Si (1.1 eV) and a band gap of GaAs (1.4 eV). Accordingly, a break-down voltage of a GaN-based HEMT is high, and it is expectable as a high break-down voltage electronic device for a motor vehicle and so on.
On the other hand, a body diode inevitably exists in a Si-based field-effect transistor. The body diode is connected in inverse parallel to the transistor, and has enough surge resistance by causing avalanche breakdown even when a die voltage (surge) for a short time occurs. However, a body diode like that of the Si-based field-effect transistor does not inevitably exist in the GaN-based HEMT, and troubles and so on may occur when a surge occurs. Conventionally, a varistor, an RC surge absorption circuit, and so on are used as a surge countermeasure element.
However, a large capacitance is parasitic on the surge countermeasure element, and therefore, there are cases when heat generation is large, and operation of a HEMT is slow. The large heat generation leads to deterioration of operation efficiency, and a delay of an operation speed of a HEMT leads to a switching loss at a switching element. Besides, a shoot-through current is easy to flow in the element at a normal operation time of a HEMT, and therefore, a power consumption is large. Further, operation speed of a HEMT is faster than the surge countermeasure element, and therefore, there is a case when a current flows in a HEMT before a surge countermeasure element operates even if a surge countermeasure element is used.